This invention relates to a device for coupling a laser beam to a transmission medium, such as an optical fiber.
As a result of recent progress in GaAs.GaA1As heterojunction lasers, it has become possible to readily supply a laser element having a total life of over ten thousand hours with an optical output of several miliwatts in continuous-wave operation at room temperature. This has rendered applications of a semiconductor laser element to optical communication and others more prosperous and practical.
In an optical communication system, an optical fiber is used as a transmission medium for transmitting a laser beam that carries information to be transmitted. An optical fiber may comprise a light guide of quartz or glass and a sheath of a synthetic resin, such as a polyamide resin known as nylon, that covers the guide. Alternatively, the light guide may be a glass fiber that has a refractive index varied along the radius and is known in the art as a "Selfoc" fiber by a trademark "Selfoc" of Nippon Selfoc Company, Limited, of Tokyo, Japan. The laser beam is caused to enter an optical fiber at an end surface thereof.
In such an optical communication system, it is highly desirable to couple a laser beam to an end surface of an optical fiber with a highest possible efficiency. Use of a lens in a device for carrying out the coupling is already known. The lens makes a divergent laser beam emitted by a laser element focus on the end surface that has a very narrow area, such as about 60 microns in diameter.
Describing more in detail, a portion of the laser beam incident on the optical fiber end surface is inevitably reflected by the end surface to reduce the efficiency of the coupling. In addition, the reflected portion of the laser beam returns to the laser element to render the operation thereof unstable. This results in an increase in noises undesiredly superposed on the information being transmitted and unavoidable variations or fluctuations in both static and dynamic characteristics of the laser element, such as the input current versus optical output characteristics, and accordingly in the modulation characteristics. The reflected portion of the laser beam more adversely affects the laser operation when the coupling is stronger. The reflection is therefore fatal to optical communication.
Speaking generally of the semiconductor laser elements, it is very often desirable that a laser beam emitted by such a laser element be coupled to a transmission medium through a lens. A laser element accompanied by a lens at a portion of its housing or package is already known. A laser device of the type accompanied by a lens as an entirety is, however, bulky and inconvenient to handle.